Hydrostatic transmission

ABSTRACT

A transmission for a vehicle includes a fixed displacement pump operable to produce a high pressure flow and a fixed displacement motor. A fluid flow path is disposed between the pump and the motor. The fluid flow path is integrated into a housing containing at least one of the pump and the motor. A single control valve controls the direction and the speed of the motor.

RELATED APPLICATION DATA

This application claims benefit under 35 U.S.C. Section 119(e) ofco-pending U.S. Provisional Application No. 60/821,443, filed Aug. 4,2006, which is fully incorporated herein by reference.

BACKGROUND

The present invention relates to hydrostatic transmissions.

Hydrostatic transmissions are commonly used in lawn tractors and othersmall vehicles to cause movement of the drive wheels. Hydrostatic drivetrains often typically include a number of components, such as ahydraulic motor, a reduction unit, a clutch unit, an oil pump and an oilreservoir.

SUMMARY

In one embodiment, the invention provides a transmission for a vehicle.The transmission includes a fixed displacement pump operable to producea high pressure flow and a fixed displacement motor. A fluid flow pathis disposed between the pump and the motor. The fluid flow path isintegrated into a housing containing at least one of the pump and themotor. A single control valve controls the direction and the speed ofthe motor.

In another embodiment, the invention provides a hydrostatic transmissionfor a vehicle. The hydrostatic transmission includes a fluid pump, afluid motor, and a rotary control valve configured to oscillateindependent of the movement of the pump and the motor. A single controlinterface is configured to control both a speed and a direction of themotor.

In yet another embodiment, the invention provides a transmission thatincludes a pump configured to discharge a flow of fluid at a firstpressure, a motor configured to rotate in response to a flow of fluid ina first flow path, and a first valve movable to vary the flow of fluidin the first flow path. A second valve is movable between a firstposition and a second position at least partially in response to thefirst pressure to direct a portion of the flow of fluid to the firstflow path and a remainder of the flow of fluid to a second flow path. Asthe valve moves toward the second position, additional flow is divertedfrom the second flow path to the first flow path to increase the speedof the motor.

In another embodiment, the invention provides a hydrostatic transmissionmodule for use with a small engine. The hydrostatic transmission moduleincludes a module housing having a first chamber and a second chamber influid communication with the first chamber. A fixed displacement pump isdisposed in the first chamber of the module housing. The fixeddisplacement pump is operatively coupled to a crankshaft of the engineand is configured to discharge a flow of fluid having a first pressure.A fixed displacement motor is disposed in the second chamber of themodule housing. The motor is configured to rotate in response to theflow of fluid from the pump. A first valve is operable to divide a flowof fluid from the pump into a first flow of fluid that flows to themotor and a second flow of fluid that flows to a sump. A second valve isdisposed in the first flow and is movable to vary the first flow offluid to the motor.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a riding lawn tractor including ahydrostatic transmission module and differential according to oneembodiment of the present invention.

FIG. 2 is a perspective view of the hydrostatic transmission module anddifferential shown in FIG. 1.

FIG. 3 is a top perspective view of the hydrostatic transmission moduleshown in FIG. 1.

FIG. 4 is a top perspective view of the hydrostatic transmission moduleshown in FIG. 3 with a housing cover removed.

FIG. 5 is a front perspective view of the hydrostatic transmissionmodule shown in FIG. 4.

FIG. 6 is a sectional view of the hydrostatic transmission module takenalong line 6-6 of FIG. 4, and illustrating a fixed displacement pump.

FIG. 7A is a sectional view of the hydrostatic transmission module andmotor taken along line 7A-7A of FIG. 5, and illustrating high pressurefluid passage.

FIG. 7B is a sectional view of the hydrostatic transmission module takenalong line 7B-7B of FIG. 5, and illustrating a flow compensating valve.

FIG. 8A is a sectional view of the hydrostatic transmission module takenalong line 8-8 of FIG. 4, and illustrating a fixed displacement motor.

FIG. 8B is an enlarged view of the flow compensating valve shown in FIG.8A, the flow compensating valve in a normal pressure mode.

FIG. 8C is an enlarged view of the flow compensating valve shown in FIG.8A, the flow compensating valve in a low pressure mode.

FIG. 9 is an exploded view of the fixed displacement motor and rotarycontrol valve shown in FIG. 8A.

FIG. 10 is a front view of a rotary plate of the rotary control valve.

FIG. 11 is a front view of a stationary plate of the rotary controlvalve.

FIG. 12 is a hydraulic circuit diagram of a hydrostatic transmissionaccording to one embodiment of the present invention illustrating fluidflow through the hydrostatic transmission when operating in a forwarddirection.

FIG. 13 illustrates the rotary control valve in a neutral position.

FIGS. 14A and 14B illustrate the rotary control valve in a partiallyopen position and show fluid flow through the rotary control valve whenoperating in a forward direction.

FIG. 15 is a hydraulic circuit diagram of a hydrostatic transmissionaccording to one embodiment of the present invention illustrating fluidflow through the hydrostatic transmission when operating in a reversedirection.

FIGS. 16A and 16B illustrate the rotary control valve in a partiallyopen position and show fluid flow through the rotary control valve whenoperating in a reverse direction.

FIG. 17 is a top perspective view of a hydrostatic transmission housingaccording to one embodiment of the present invention.

FIG. 18 is a rear perspective view of the hydrostatic transmissionhousing shown in FIG. 17.

FIG. 19 is a perspective view of an end of the drive shaft shown in FIG.2, in particular an end of the drive shaft coupled to the hydrostatictransmission.

FIG. 20 is a perspective view of a bushing used to interconnect a motoroutput shaft and a drive shaft.

FIG. 21 is a perspective view of a differential according to oneembodiment of the invention.

FIG. 22 is a perspective view of the differential shown in FIG. 21 witha portion of the housing removed.

FIG. 23 illustrates an arrangement of gears within the differentialshown in FIG. 22.

FIG. 24 illustrates an operator interface and linkage for controllingthe rotary control valve, according to one embodiment of the invention.

FIG. 25 illustrates an operator interface and linkage for controllingthe rotary control valve, according to one embodiment of the invention.

FIG. 26 illustrates an operator interface and linkage for controllingthe rotary control valve, according to one embodiment of the invention.

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting.

DETAILED DESCRIPTION

FIG. 1 illustrates a riding lawn tractor 10 including a hydrostatictransmission module 14 according to one embodiment of the invention, anda differential 18 interconnected with the hydrostatic transmissionmodule 14 by a drive shaft 22. Although the illustrated hydrostatictransmission module 14 is used with the riding lawn tractor 10 in FIG.1, it should be readily apparent to those of skill in the art that thehydrostatic transmission module 14 may be used with other vehicles thatemploy small engines (i.e., generally one or two cylinder engines). Thelawn tractor 10 includes a steering wheel 26, a directional lever 28, adriver's seat 30 and an engine 34 mounted on a vehicle frame 38. Theframe 38 is supported by front wheels 42 and driven rear wheels 46, anda mower deck 50 is mounted to an underside of the frame 38.

FIG. 2 illustrates the engine 34, the hydrostatic transmission module14, and the differential 18. Referring to FIGS. 1 and 2, the engine 34is a vertical output shaft type having a vertical crankshaft (not shown)to which an output pulley 54 is interconnected via the hydrostatictransmission module 14 (see also FIG. 6). The crankshaft of the engine34 is coupled to and mounted co-axially with a pump shaft 58 (FIG. 3),or input shaft, of the hydrostatic transmission module 14. In theillustrated embodiment, the crankshaft extends through the pump shaft 58to engage the output pulley 54, as discussed below. The output pulley 54is generally coupled to an input pulley on the mower deck 50 by a drivebelt 56, as is known in the art. Driving power from the engine 34 istransmitted from the output pulley 54 to the input pulley (not shown) todrive one or more mower blades supported beneath the mower deck 50.Although the illustrated hydrostatic transmission module 14 isconfigured for use with a vertical output shaft type engine, it shouldbe readily apparent to those of skill in the art that the hydrostatictransmission may be configured for use with a horizontal output shafttype engine.

One end of the drive shaft 22 is coupled to and mounted co-axially to amotor shaft 62 (FIG. 3), or output shaft, of the hydrostatictransmission module 14, and an opposite end of the drive shaft 22 iscoupled to the differential 18. The differential 18 contains one or moreaxle shafts to which the rear wheels 46 are attached. The differential18 allows power to be transmitted to both rear wheels 46 during a turneven though one of the wheels will travel a greater distance, andtherefore must rotate faster, during the turn than the other wheel.

FIG. 3 illustrates one embodiment of the hydrostatic transmission module14, which integrates and houses all the components for a hydrostatictransmission in a single housing. The single housing enables modularityof the hydrostatic transmission and a transmission design that may beused with a variety of vehicle and engine types as a packaged unit. Byintegrating all the components of the hydrostatic transmission, themodule provides a common housing for storing components of thehydrostatic transmission, which decreases the difficulty and cost ofmanufacturing, increases durability of the hydrostatic transmission, anddecreases leakage of the hydrostatic transmission.

The hydrostatic transmission module 14 includes a housing 66, a firstremovable cover 70, and a second removable cover 74. The first cover 70encloses a first chamber 78 (FIG. 4) of the housing 66 and includes anopening 82 for allowing the pump shaft 58 of the hydrostatictransmission 14 to pass. The second cover 74 encloses a second chamber86 (FIG. 18) of the housing 66 and includes an opening 90 for allowingthe motor shaft 62 of the hydrostatic transmission 14 to pass. Bothcovers 70, 74 are secured to the housing by bolt fasteners 94, althoughother conventional fasteners may be used, such as screws, pins, clips,clamps, inter-engaging elements, or the like.

FIGS. 4 and 5 illustrate the hydrostatic transmission module 14 with thefirst cover 70 removed to reveal the first chamber 78 of the module 14.A seal 96 is positioned between the cover 74 and the housing 66 tocreate a fluid-tight seal therebetween. In the hydrostatic transmissionmodule 14, components of the hydrostatic transmission are integratedinto the common housing 66 and the housing 66 includes an integrallyformed fluid flow path (FIGS. 12 and 15). The integrally formed fluidflow path eliminates a complicated hydraulic circuit to coordinatefunction between components and improves fluid flow between components.Further, the fluid flow path improves durability of the hydrostatictransmission and decreases leakage because no separate, unpackagedcomponents are utilized and the fluid flow path is self-contained withinthe housing. Positioned within the first chamber 78 of the hydrostatictransmission module 14 are components of the hydrostatic transmission,including the pump shaft 58, a fixed displacement pump 98, a filter 102,a high pressure fluid passage 106 with a pressure relief valve 110(shown in FIG. 7A), a flow compensating valve 114 (shown in FIG. 7B),and a bevel directional gear 118. In some constructions, and asdiscussed below, a pump housing portion 122, the high pressure fluidpassage 106, a valve housing chamber 198 (shown in FIG. 5) and a valvehousing portion 274 are preferably all integrally formed with thehousing 66 such that all of these components are a single piece.

FIGS. 17 and 18 illustrate the housing 66 of the hydrostatictransmission module 14 according to one embodiment of the invention. Thehousing 66 includes a first outer wall 119 that defines the firstchamber 78 of the housing 66, and a generally round second outer wall120 that partially defines the second chamber 86 and the valve housingportion 274. A rear section of the valve housing portion 274 (FIG. 18)extends into the first chamber 78 of the housing 66, however, a housingwall 270 and a circumferential wall 121 separate the two chambers 78,86. An exposed edge of the first outer wall 119 includes apertures 158for receiving fasteners 94 to secure the first cover 70 to the housing66. An exposed edge of the second outer wall 120 includes apertures 158for receiving fasteners 94 to secure the second cover 74 to the housing66.

The common housing of the hydrostatic transmission module integrates allthe components of the hydrostatic transmission into one packaged unit,which improves efficiency and cost of manufacturing and assembling thehydrostatic transmission because all the components are located withinone housing. In addition, fastening apertures, valve housing portions,chambers, fluid passageways, and fluid ports are formed as part of thehousing, thereby eliminating the need of additional pieces andstructure. The common housing and integrally formed fluid flow path alsoeliminate the need for a complicated hydraulic circuit between variouscomponents.

Referring to FIGS. 4, 5, 17 and 18, within the first chamber 78, thehousing 66 defines a pump housing portion 122 having a generallycylindrical shape for housing the pump 98. The pump housing portion 122includes a cylindrical outer wall 126 and a lower surface defined by abottom wall 130 of the housing 66. An oil inlet port 134 (FIG. 17) thatfluidly communicates with the first chamber 78 and an oil outlet port138 (FIG. 17) that fluidly communicates with the high pressure fluidpassage 106 are formed in the outer wall 126. The filter 102 ispositioned at the oil inlet port 134 to filter oil as it enters the pump98. The outer wall 119 of the housing 66 may include an opening 140positioned adjacent the oil inlet port 134 of the pump housing portion122. The opening 140 may be used for inserting the filter 102 (FIG. 5)into the first chamber 78 for communication with the oil inlet port 134.The opening 140 is used to allow for the passage of a tool duringmanufacture of the housing 66 and is plugged after fabrication (FIG. 5).The tool is used to complete formation of the oil inlet port 134.

The pump housing portion 122 includes a center hole 142 (FIG. 17) forreceiving the pump shaft 58, and chamber walls 146 extending generallyupward from the bottom wall 130. The chamber walls 146 define an inletchamber 150 for receiving oil from the first chamber 78 through the oilinlet port 134, and a discharge chamber 154 for discharging oil throughthe oil outlet port 138 into the high pressure fluid passage 106. In oneconstruction, the oil outlet port 138 is formed by drilling an opening156 (FIG. 7A) through the outer wall 119 of the housing 66, which isplugged after fabrication. In the illustrated embodiment, the chamberwalls 146 have a height less than a height of the outer wall 126 foraccommodating the pump 98. An exposed edge of the outer wall 126includes apertures 158 (FIG. 17) for receiving fasteners 94 to securethe first cover 70 to the housing 66.

In operation, the hydrostatic transmission module transmits drivingpower from the engine 34 to the differential 18 to drive the rear wheels46 at a vehicle speed selected by an operator. The hydrostatictransmission module 14 pumps hydraulic fluid, in particular oil, withthe pump 98 from the first chamber 78 of the housing 66 to a fixeddisplacement motor 162 (see FIG. 8A) housed within the second chamber 86of the housing 66. The flow compensating valve 114 operates to maintainconstant speed of the motor 162, and in particular the drive shaft 22coupled to the differential 18, by controlling oil flow through themotor 162. Output demands of the motor 162 vary as the vehicle 10travels over uneven terrain. Based on the output demands, the flowcompensating valve 114 operates to control fluid flow to the motor 162and along a bypass flow path away from the motor 162 to maintain aconstant speed of the vehicle, as discussed below. In the illustratedembodiment, the flow compensating valve 114 is positioned in parallelwith the rotary control valve 254 (FIGS. 8A, 12 and 15) to reduce thenumber of orifices in the hydraulic circuit between the pump 98 and themotor 162, which could create a pressure drop and decrease power trainefficiency. Further, the flow compensating valve 114 is easily packagedwithin the common housing 66 upstream of the control valve.

FIG. 6 is a sectional view of the fixed displacement pump 98 and theoutput pulley 54, which is coupled to and mounted co-axially with theengine crankshaft, via a clutch assembly 166 for the mower blades. Insome constructions, the clutch assembly 166 is eliminated and the outputpulley 54 is connected to an extended engine crankshaft that passesthrough pump shaft 58. Alternatively, the pulley 54 could be mounteddirectly to a solid pump shaft 58. It should be readily apparent tothose of skill in the art that the engine crankshaft may be mountedco-axially to a solid pump shaft 58 and the pump shaft 58 is therebymounted co-axially to the clutch assembly 166. The pump 98 operates as asump pump within the hydrostatic transmission module 14 to pump oil froma hydraulic fluid reservoir (i.e., sump) defined by the first chamber 78to the fixed displacement motor 162.

Referring to FIGS. 4-6, the pump 98 is a positive displacement gerotorpumping unit housed within the pump housing portion 122. The pump 98includes an inner drive rotor 170 and an outer driven rotor 174, eachrotor 170, 174 including a plurality of teeth that mesh and unmesh asthe rotors rotate. The outer rotor 174 includes one more tooth than theinner rotor 170. The teeth unmesh on a suction side of the pump 98 toallow atmospheric pressure to force oil into the inlet chamber 150 ofthe pump housing portion 122 from the sump 78 through the oil inlet port134. The teeth mesh on a discharge side of the pump 98 to force oil outof the outlet chamber 154 (FIG. 17) of the pump housing portion 122through the oil outlet port 138 (FIG. 17) to the high pressure fluidpassage 106. An outer ring 178 surrounds an outer circumference of theouter rotor 174 to allow the outer rotor 174 to rotate with respect tothe pump housing portion 122. The positive displacement gerotor pumpingunit 98 provides a pump that is more simple, less bulky, and costs lessand is easier to manufacture than other commercially available fixeddisplacement pumps. The gerotor pump 98 operates as is known in the art,and it should be readily apparent to those of skill in the art thatother known pumps and fixed displacement pumps may be used to performthis function, such as external gear, internal gear, vane, axial piston,and radial piston type pumps.

The pump shaft 58 extends through a center opening 182 defined by theinner rotor 170 of the pump 98 and the center hole 142 of the pumphousing portion 122. The pump shaft 58 is integrally splined to engagethe inner rotor 170 of the pump 98, such that rotation of the pump shaft58 drives the inner rotor 170 to operate the pump 98. A first seal 186is positioned between the pump shaft 58 and the lower housing portion130 to provide a fluid-tight seal, and a second seal 190 is positionedbetween the pump shaft 58 and the first cover 70 to provide afluid-tight seal.

The clutch assembly 166, or clutch/brake assembly, as it is sometimesknown in the art, includes an inner hub 194 (i.e., an input hub) that ismounted co-axially to the pump shaft 58 by a bolt, or other knownfastener. The clutch assembly 166 is positioned on an underside of thehydrostatic transmission module 14 and is used to control the mowerblades. The output pulley 54 is coupled to the clutch assembly 166, orclutch/brake assembly, as known in the art, by a bearing and receives adrive belt 56 (FIG. 1).

Referring to FIGS. 4, 5, 7A, 17 and 18, within the first chamber 78, thehousing 66 also defines the high pressure fluid passage 106 integrallyformed by the housing 66 and a valve housing chamber 198 for housing theflow compensating valve 114. A first end 106A of the high pressure fluidpassage 106 extends through the outer wall 119 of the housing 66. Thepressure relief valve 110 is positioned at the first end 106A of thepassage 106 for relieving pressure within the high pressure fluidpassage 106 should the pressure exceed a predetermined value. Thepressure relief valve 110 plugs the first end 106A of the passage 106and oil is relieved through a relief passage 106B into the first chamber78. A second end 106C of the high pressure fluid passage 106 fluidlycommunicates with the motor 162 positioned within the second chamber 86via an inlet port 202 (FIG. 7B), or valve inlet. The inlet port 202 isdefined by the housing 66 and is positioned under the valve housingchamber 198. In the illustrated embodiment, the second end 106C of thehigh pressure fluid passage 106 is at least partially formed by drillinga hole through the housing. The hole is then plugged with a check valve199 during fabrication (FIG. 7A). The pressure relief valve 110 ispositioned in the high pressure passage 106 between the pump 98 and themotor 162 because such a location facilitates machining operations ofthe common housing 66 and assembly operations of the hydrostatictransmission module 14, as well as drains directly into the firstchamber 78, i.e., the hydraulic fluid reservoir.

The check valve 199 opens in response to a pressure in the first chamber78 that is in excess of the pressure within the high pressure passage106. This condition can occur when the motor 162 is rotated without oilflow, such as what occurs when a user pushes a tractor equipped with thetransmission. The movement of the motor 162 causes it to pump oil out ofthe high pressure passage 106, thereby reducing the pressure. The checkvalve 199 opens during this condition to allow the motor 162 to turnwithout excessive force and to inhibit the intake of outside air throughthe motor shaft seals that would undesirably aerate the oil.

Referring to FIGS. 5, 7B, and 8A, the valve housing chamber 198 isgenerally cylindrical and includes an outer wall 206 defining an innerchamber 210 for receiving the flow compensating valve 114. The innerchamber 210 includes a first end 210A proximate the directional gear 118and a plugged second end 210B. Two ports 214, 218 extend through theouter wall 206 between the first chamber 78 and the inner chamber 210.The first port 214 is positioned at the first end 210A of the innerchamber 210 and provides a passageway 230, which connects the first port214, the inner chamber 210, and the high pressure fluid passage 106. Thefirst port 214 is plugged after the passageway 230 is drilled (asillustrated in FIG. 7B). The passageway 230 communicates fluid pressurefrom the high pressure fluid passage 106 to the flow compensating valve114. The second port 218, which may perform a metering function,provides a portion of a bypass flow path from the high pressure fluidpassage 106, through the flow compensating valve 114 to the firstchamber 78 of the housing 66.

The flow compensating valve 114 is utilized to maintain a constant speedof the lawn tractor 10, and more particularly the motor 162 and thedrive shaft 22. The flow compensating valve 114 maintains a fixed flow,or constant speed, by varying bypass flow around the motor 162. The flowcompensating valve 114 is partially controlled by a pressure signal fromthe high pressure passage 106. During operation, the lawn tractor 10travels over differing terrains and topography, but the operator desiresto maintain constant speed of the lawn tractor 10. Output demands of themotor 162 vary as the lawn tractor 10 travels over the differingterrains and topography, and the flow compensating valve 114 is utilizedto control bypass flow such that output demands are met to maintain aconstant speed.

With reference to FIGS. 8A and 7B, the flow compensating valve 114 isretained within the valve housing chamber 198 of the hydrostatictransmission module 14 and includes a spool-type valve 234, a biasingspring 238, and a plug 242. The spool 234 is positioned within the innerchamber 210 of the valve housing chamber 198. A first end 234A of thespool 234 has a first diameter substantially equal to a diameter of theinner chamber 210, and defines a piston 234A. A rod portion 234B extendsrearward from the piston 234A and has a second diameter less than thefirst diameter. The spool 234 includes a stop portion 234C positionedbetween the rod portion 234B and a travel limiter 234D that can beformed as part of the spool 234 or separated from the spool 234 toinhibit excess travel of the spool 234. The plug 242 plugs the secondend 210B of the inner chamber 210 and retains the spring 238 in theinner chamber 210. The spring 238 is positioned between the stop portion234C of the spool 234 and the plug 242 in the second end 210B.

The spring 238 biases the flow compensating valve 114 toward a firstposition, as shown in FIG. 7B, in which the piston 234A is positioned atthe first end 210A of the inner chamber 210 such that port 218 isclosed. The spool 234 is exposed to high pressure oil at the extreme endadjacent the piston 234A such that the high-pressure oil works to movethe spool 234 against the biasing force of the spring 238. Thehigh-pressure oil flows to the end of the piston from the high pressurefluid passage 106 via passageway 230 and port 214.

With reference to FIG. 8A, high-pressure oil is also directed from thehigh pressure fluid passage 106 to the space between the piston 234A andthe stop portion 234C. Because this high-pressure oil acts on twoequally-sized surfaces, it has little effect on movement of the spool234. The space opposite the piston 210B is in fluid communication withhigh-pressure oil from a source adjacent the rotary control valve 254.Specifically, oil is directed from the rotary control valve 254 throughpassageway 246 to the second end 210B. The pressure of this oil can varywith operation, thus controlling movement of the spool 234, as will bediscussed.

For example, when this oil is at a pressure similar to the high-pressureoil from the pump, the forces produced by the oil on the spool 234 aresubstantially balanced such that the spool is biased by the spring 238into the position illustrated in FIGS. 7B and 8A. However, duringcertain conditions, the pressure in the second space 210B can drop. Forexample, when the rotary control valve 254 is positioned in a neutralposition, there is little flow to the motor 168 and the fluidcommunication path to the second space 210B is opened to the first space78. With this pressure released, the high-pressure on the piston 234Aovercomes the biasing force of the spring 238 and moves the spool towardthe position illustrated in FIG. 8B. In this position, the space betweenthe piston 234A and the stop portion 234C provides for fluidcommunication between the high-pressure fluid passage 106 and the bypassport 218. Thus, oil travels directly from the high-pressure fluidpassage 106 to the second or bypass port 218 and into the sump 78without traveling through the rotary control valve 254 or the motor 162.

When output demands of the motor 162 increase, such as when a vehicleemploying the transmission climbs a hill, additional oil must beprovided to the motor 162 to maintain the desired speed. As the vehiclebegins to climb the hill, the motor 162 slows. The slower rotationreduces the amount of oil that can pass through the motor 162 andincreases the pressure of the oil in the second space 210B. Thus, thespool 234 is biased toward the position illustrated in FIG. 7A and theflow area between the high-pressure passage 106 and the bypass port 218is reduced. Thus, more oil is delivered to the motor 162 to speed therotation and maintain the desired speed up the hill. Eventually, thespool 234 moves to a 100 percent demand position in which there is nobypass flow and the motor 162 uses all of the available oil.

FIG. 8B illustrates the flow compensating valve 114 in a normal pressure(P1) position when the lawn tractor 10 is operating on level terrain andsome oil is bypassing the motor 162. Normal pressure will vary dependingon gross vehicle weight, ground conditions, and other factors. In oneexample, normal pressure is about 200 psi. When the flow compensatingvalve 114 is in the normal pressure position, the normal (level terrain)load creates approximately normal pressure P1 in the high pressure fluidpassage 106 and the flow compensating valve 114 will allow about 50percent of the oil delivered by the pump 98 to bypass the motor 162.

FIG. 8C illustrates the flow compensating valve 114 in a high pressure(P2) position when the lawn tractor 10 is operating on an uphill grade.When the flow compensating valve 114 is in the high pressure position,the high (uphill terrain) load creates high pressure P2 in the secondend 210B and the flow compensating valve 114 moves to reduce bypass flowfrom the fluid passage 106 to the housing chamber 78. A constant speedof the lawn tractor 10 and the motor 162 will be maintained byincreasing flow to the motor, and thereby decreasing bypass fluid flow.

As discussed, FIG. 8B illustrates a normal pressure position and FIG. 8Cillustrates a high pressure position. As one of ordinary skill in theart will realize, the valve can move to a low pressure position in whichadditional flow bypasses the motor 162. For example, the flowcompensating valve 114 will move to a low pressure (P3) position whenthe lawn tractor 10 is operating on a downhill grade. When the flowcompensating valve 114 is in the low pressure position, the low(downhill terrain) load creates low pressure P3 in the second space 210Band the flow compensating valve 114 moves to allow additional bypassflow up to about 100 percent (i.e., neutral) of the flow provided by thepump 98. A constant speed of the lawn tractor 10 and the motor 162 willbe maintained by increasing motor bypass flow, and thereby decreasingfluid flow to the motor 162. It should be readily apparent to those ofskill in the art that the flow compensating valve 114 can block anypercentage of the bypass flow depending on system demands.

Referring to FIGS. 5 and 7B, the housing 66 defines a slot 250 forreceiving the bevel directional gear 118, an outer circumference ofwhich is intermeshed with the rotary control valve 254 positioned in thesecond chamber 86. The directional gear 118 includes a central bore 258for receiving a control shaft 262. The shaft 262 is utilized to selectspeed and forward or reverse operation of the motor 162 via the rotarycontrol valve 254, and thereby of the lawn tractor 10. A portion 262A ofthe shaft 262 positioned outside the hydrostatic transmission module 14is interconnected with an operator linkage to select an operatingdirection of the lawn tractor 10, such as the directional lever 28 (FIG.1). The shaft 262 extends through an opening 264 in the outer wall 119of the housing 66 (and is supported by a journal in the housing 66),through the central bore 258 of the directional gear 118, and anotherportion 262B of the shaft 262 is rotatably anchored within a journal 266in the housing 66. A seal 263 provides a fluid-tight seal between theshaft 262 and the housing 66.

Rotation of the control shaft 262 by an operator (via an operatorinterface and control linkage) in a first direction causes rotation ofthe directional gear 118 in a first direction, which causes forwardoperation of the rotary control valve 254, as discussed below. Rotationof the control shaft 262 by an operator (via an operator interface andcontrol linkage) in a second direction causes rotation of thedirectional gear 118 in a second direction, which causes reverseoperation of the rotary control valve 254, as discussed below.Specifically, rotation of the control shaft 262 produces a correspondingoscillation, rotation, revolution, etc. of the rotary control valve 254through an angle less than about 180 degrees. In preferredconstructions, the rotary control valve 254 rotates through an anglebetween about 40 and 80 degrees. In other preferred constructions, therotary control valve 254 rotates through an angle between about 20 and40 degrees. The operator interface and linkage controls direction andspeed of the motor by rotating the control valve 254 in either a firstdirection or a second direction. Examples of an operator interface andcontrol linkage that could be used to control the directional gear 118are a push/pull cable with fender-mounted shifter (FIG. 24), amechanical linkage with fender-mounted shifter (FIG. 25), and amechanical linkage with right-side foot pedal (FIG. 26). As discussedbelow, each of these embodiments includes a linkage that couples to thecontrol shaft 262 to reciprocate the control valve 254 in eitherdirection.

FIG. 24 illustrates one embodiment of an operator interface and linkage600 for controlling the rotary control valve 254 including afender-mounted shifter 604, a push/pull cable 608 having one end coupledto the shifter 604, and a linkage 612 connecting an opposite end of thecable 608 and the control shaft 262. An operator controls direction oftravel by pushing the shifter 604 forward or pulling the shifterbackwards. In the illustrated embodiment, the shifter 604 starts in aneutral position. By pushing the shifter 604 forward, the cable 608pushes the linkage 612 to rotate the control shaft 262, and thereby thedirectional gear 118, in a first direction. By pulling the shifter 604backwards, the cable 608 pulls the linkage 612 to rotate the controlshaft 262, and thereby the directional gear 118, in a second direction.

FIG. 25 illustrates another embodiment of an operator interface andlinkage 616 for controlling the rotary control valve 254 including afender-mounted shifter 620, a first linkage 624 having one end coupledto the shifter 620, and a second linkage 628 connecting an opposite endof the first linkage 624 and the control shaft 262. An operator controlsdirection of travel by pushing the shifter 620 forward or pulling theshifter backwards. In the illustrated embodiment, the shifter 620 startsin a neutral position. By pushing the shifter 620 forward, the firstlinkage 624 pushes the second linkage 628 to rotate the control shaft262, and thereby the directional gear 118, in a first direction. Bypulling the shifter 620 backwards, the first linkage 624 pulls thesecond linkage 628 to rotate the control shaft 262, and thereby thedirectional gear 118, in a second direction.

FIG. 26 illustrates another embodiment of an operator interface andlinkage 632 for controlling the rotary control valve 254. The operatorinterface and linkage 632 includes a two-directional foot pedal 636mounted to the vehicle frame 38, a first linkage 640 connected to thefoot pedal 636, a second linkage 644 connected to the first linkage 640,and a third linkage 648 connected between the second linkage 644 and thecontrol shaft 262. In the illustrated embodiment, the foot pedal 636 isgenerally L-shaped and includes a first lever member 636A for effectingtravel of the vehicle in a first direction, a second lever member 636Bfor effecting travel of the vehicle in a second direction, and an arm652 for coupling the pedal 636 to the first linkage 640. FIG. 26 showsthe pedal in a neutral position. Pressing down on the first lever member636A of the pedal 636 causes the pedal 636 and arm 652 to rotate in afirst direction, which causes the interconnected linkages 640, 644, 648to rotate the control shaft 262, and thereby the directional gear 118,in a first direction. Pressing down on the second lever member 636B ofthe pedal 636 causes the pedal 636 and arm 652 to rotate in a seconddirection, which causes the interconnected linkages 640, 644, 648 torotate the control shaft 262, and thereby the directional gear 118, in asecond direction. In a further embodiment, two pedals may be used toselect the travel direction of the vehicle, or the pedal linkages mayinclude a spring member to return the pedal to a neutral position whenan operator's foot has been removed from the pedal.

It should be readily apparent to those of skill in the art that infurther embodiments of the operator interfaces and linkages, operationof the interface in a first direction may initiate operation of thevehicle in a reverse direction and operation of the interface in asecond direction may initiate operation of the vehicle in a forwarddirection.

FIG. 8A illustrates the rotary control valve 254 and the motor 162housed in the second chamber 86 of the hydrostatic transmission module14, and FIG. 9 illustrates an exploded view of the rotary control valve254 and the motor 162. The rotary control valve 254 provides bothdirectional control (forward-neutral-reverse) and flow control (meteredoutlet ports) in the hydraulic circuit. The rotary control valve 254 iseasy to manufacture, integrate with the motor 162, and integrate withthe housing 66. Further, the rotary control valve 254 has a flexibleporting design such that the ports may be custom shaped and tunedthrough the valve 114 to achieve desired flow characteristics, shiftquality and drivability. In another embodiment, an integrated linearspool valve may be utilized with the hydraulic transmission module 14 tocontrol direction and fluid flow with respect to the motor 162.

As seen in FIGS. 4 and 5, the second chamber 86 of the housing 66 isseparated from the first chamber 78 by the housing wall 270, and thehousing 66 includes the valve housing portion 274 for housing a portionof the rotary control valve 254. The housing wall 270 of the valvehousing portion 274 separates the first chamber 78 and the secondchamber 86, and is partially defined by the valve housing chamber 198for the flow compensating valve 114.

Referring to FIG. 18, the housing wall 270 includes a close signalorifice 279, the valve inlet 202 between the high pressure fluid passage106 and the second chamber 86, and a valve outlet 278 having a notch 281between the second chamber 86 and the first chamber 78. Further, asillustrated in FIG. 8A, the valve housing portion 274 includes anopening 274A adjacent the slot 250 for allowing the directional gear 118to intermesh with the rotary control valve 254.

As oil is discharged from the rotary control valve 254 through the valveoutlet 278, the oil is deflected downward toward the bottom wall 130 ofthe first chamber 78 and away from the pump 98. Referring to FIGS. 5 and7A, the oil is deflected downward by a portion 280 of the housing 66defining the high pressure fluid passage 106. The portion 280, orbaffle, of the housing 66 is spaced apart from and aligned with thevalve outlet 278 formed in the wall 270 of the valve housing portion274. A partition wall 281 extends between the outer wall 119 of thehousing 66 and the outer wall 126 of the pump housing portion 122. Thepartition wall 281 defines a flow path (shown by arrows in FIG. 17) foroil to flow in the sump 78, and in particular to prevent oil recentlydischarged from the rotary control valve 254 from flowing directly backto the oil inlet port 134 and the pump 98 and to allow the oil tode-aerate.

As illustrated in FIGS. 8A and 9, the rotary control valve 254 includesa rotary plate 282 (FIG. 10) and a stationary plate 286 (FIG. 11). Therotary plate 282 is positioned between the housing wall 270 and thestationary plate 286, and within the valve housing portion 274. An outercircumference of the rotary plate 282 includes gear teeth 306 forintermeshing with the bevel directional gear 118 through the opening274A adjacent the slot 250. Rotation of the directional gear 118 in thefirst direction causes the rotary plate 282 to rotate in a firstdirection (counter-clockwise in the illustrated embodiment) to therebyfacilitate forward operation of the motor 162, as discussed below.Likewise, rotation of the directional gear 118 in the second directioncauses the rotary plate 282 to rotate in a second direction (clockwisein the illustrated embodiment) to thereby facilitate reverse operationof the motor 162, as discussed below. In the illustrated embodiment, therotary plate 282 has a diameter smaller than a diameter of thestationary plate 286. In one embodiment, the rotary plate 282 is formedfrom a single sintered metal piece or metal casting, and the gear teeth306 may be formed separately from a plastic material. In still otherconstructions, the rotary plate 282 and gear teeth 306 are integrallyformed as a single piece. The directional gear 118 may be formed of asingle piece sintered metal, metal casting, or molded plasticconstruction.

With reference to FIG. 10, the rotary plate 282 includes a centeropening 310 aligned with a central axis of the plate 282. The centeropening 310 faces the housing wall 270, but does not pass completelythrough the rotary plate 282. The rotary plate 282 also includes twodirectional openings, a forward opening 314 and a reverse opening 318,centered about the radial axis and that pass through the rotary plate282. The openings 314, 318 allow oil to flow through the rotary plate282, to the motor 162 and back through the rotary plate 282, and therebyfacilitate forward or reverse operation of the motor 162 depending onwhich opening 314, 318 is utilized as an inlet opening for oil flow tothe motor 162. The openings 314, 318 are generally arc-shaped and havesubstantially identical shapes. The openings 314, 318 are positionedbetween the gear teeth 306 and the center opening 310 of the plate 282.Each opening 314, 318 also varies in width along a length of theopening. The varying width allows for variation of oil flow to the motor162 and, thereby allows for finer control of motor speed.

As illustrated in FIG. 8A, the center opening 310 is in fluidcommunication with the passageway 246 that provides fluid communicationto the second space 210B. The center opening 310 connects to atransverse passage 900 that connects to both the forward opening 314 andthe reverse opening 318. Two check valve seats 905 (shown in FIG. 8B)are formed or positioned within the transverse passage 900 with a singleball 910 positioned between the seats 905. Thus, the highest pressurefluid from either opening 314, 318 will move the ball 910 toward theopposite seat 905 such that the highest pressure is delivered to thesecond space 210B, via passage 222 and 246, but no fluid is transferreddirectly between the openings 314, 318. In this way, the correctpressure signal is delivered to the second space 210B no matter thedirection of rotation of the motor 162.

A second passage 915, illustrated in FIG. 7A, formed within the rotaryplate 282 connects the transverse passage 900, the passageway 246, andthe second space 210B to a vent opening 920. As illustrated in FIG. 7A,when in the neutral position, the vent opening 920 aligns with the notch281 to open the second space 210B to the housing chamber 78. Thisassures that when in the neutral position, no pressure will be trappedin the transverse passage 900, the passageway 246 or the second space210B. This vent opening 920 only aligns with the notch 281 when therotary plate 282 is positioned in the neutral position. In all otheroperating positions, the vent opening 920 is closed.

The stationary plate 286, illustrated in FIG. 11, is positioned betweenthe rotary plate 282 and the motor 162, and within the second chamber86. In the illustrated embodiment, the stationary plate 286 has adiameter greater than the diameter of the rotary plate 282. Thestationary plate 286 includes a bore 322 centered about the central axisof the plate 286 for receiving and supporting the motor shaft 62. Thestationary plate 286 is mounted to the motor shaft 62 at the bore 322 bya first bearing 326 to allow the motor shaft 62 to rotate relative tothe stationary plate 286. The stationary plate 286 includes twoopenings, a first opening 330 and a second opening 334, for allowing oilto flow through the stationary plate 286, to the motor 162 and backthrough the stationary plate 286. The openings 330, 334 are generallyarc-shaped and are mirror images of each other. In the illustratedembodiment, the openings 330, 334 of the stationary plate 286 are largerthan the directional openings 314, 318 of the rotary plate 282. Theopenings 330, 334 are positioned between an outer circumference and thebore 322 of the plate 286. In one embodiment, the stationary plate 286is formed from sintered metal. It should be noted that otherconstructions may vary the size and/or the shape of the forward andreverse openings 314, 318 of the rotary plate 282 and/or the openings330, 334 of the stationary plate 286 to allow for variations in speedbetween forward and reverse. As such, the invention should not belimited to constructions that include identical forward and reverseopenings 314, 318 and 330, 334.

A lower edge of the stationary plate 286 includes a notch 286A. When therotary control valve 254 is assembled in the second chamber 86, thenotch 286A is aligned with an opening 335 (shown in FIG. 7A) in theouter wall 120 of the housing 66. A pin (not shown) extends through theopening 335 and engages the stationary plate 286 at the notch 286A toprevent rotation of the stationary plate 286 in the second chamber 86.An upper edge of the stationary plate 286 includes a notch 286B, and avent channel 286C extends between the bore 322 and the notch 286B. Whenthe rotary control valve 254 is assembled in the second chamber 86, thenotch 286B is aligned with an opening 336 (shown in FIG. 7A) in theouter wall 120 of the housing 66. The vent 286C and the notch 286B ventpressure buildup behind the motor shaft seals to the reservoir 78.

As discussed above, the shape and size of the ports or openings areselected to achieve desired flow characteristics, shift quality anddrivability of the hydrostatic transmission module 14, and can be variedas required by the particular application.

Referring to FIG. 8A, the motor 162 is a positive displacement gerotormotor housed within the second chamber 86 of the hydrostatictransmission module 14 and the second cover 74 is attached to thehousing 66 to enclose the chamber 86. The positive displacement gerotormotor 162 provides a motor that is simpler, less bulky, and costs lessand is easier to manufacture than other commercially available fixeddisplacement motors. A seal 338 is positioned between the cover 74 andthe housing 66 to create a fluid-tight seal. The motor 162 is positionedbetween the rotary control valve 254 and the cover 74. The gerotor motor162 includes an inner drive rotor 342, an outer driven rotor 346, and aneccentric, outer ring 350. The rotors 342, 346 include a plurality ofteeth that mesh and unmesh as the rotors 342, 346 rotate, and the outerrotor 346 includes one more tooth than the inner rotor 342. As discussedabove with respect to the gerotor pump 98, the teeth unmesh on an inlet,or high pressure, side of the motor 162 to allow oil into the motor 162from the rotary control valve 254. The teeth mesh on a discharge side ofthe motor 162 to allow oil out of the motor 162 and back through therotary control valve 254.

The outer ring 350 surrounds an outer circumference of the outer rotor346 and allows the outer rotor 346 to rotate with respect to the housing66. In the illustrated embodiment, an inner bore of the outer ring 350is coated with an anti-friction material to reduce motor friction. In afurther embodiment, the outer ring 350 may be eliminated such that theouter rotor 346 rotates within a bore, or journal, of the housing 66,which may be coated or uncoated with an anti-friction material. Ifuncoated, an oil film may provide a hydrodynamic bearing between theouter rotor 346 and the bore surface.

The motor shaft 62 extends through a central bore 354 defined by theinner rotor 342, through the cover 74 and out of the hydrostatictransmission module 14. The cover 74 is mounted to the motor shaft 62 bya second bearing 358 to allow the motor shaft 62 to rotate relative tothe cover 74. A seal 362 is positioned between the motor shaft 62 andthe second cover 74 to provide a fluid-tight seal.

The motor shaft 62 is integrally splined to engage the inner rotor 342of the motor 162, such that rotation of the motor 162 also drives themotor shaft 62. As discussed below, the rotary control valve 254controls whether the motor 162 operates in a forward direction or areverse direction. When the rotary control valve 162 is actuated in thefirst direction, oil is pumped through the rotary control valve 254 andthe motor 162 in a first direction. Thereby, the motor 162 and the motorshaft 62 rotate in a first direction, which operates the lawn tractor 10in a forward direction. When the rotary control valve 254 is actuated inthe second direction, oil is pumped through the rotary control valve 254and the motor 162 in a second direction. Thereby, the motor 162 and themotor shaft 62 rotate in a second direction, which operate the lawntractor, or other vehicle, 10 in a reverse direction. The gerotor motor162 operates as is known in the art, and it should be readily apparentto those of skill in the art that other known motors or fixeddisplacement motors may be used to perform this function, such asexternal gear, internal gear, vane, axial piston, and radial piston typemotors.

FIG. 12 is a hydraulic circuit diagram illustrating oil flow through thehydraulic transmission module 14 during forward operation. Like elementsto those described above with respect to FIGS. 1-11 are referenced bythe same reference numerals. The circuit diagram illustrates a fluidflow path 364 through the hydrostatic transmission module, the flow path364 preferably being substantially integrally formed in the modulehousing 66. During forward operation of the module 14, when the controlshaft 262 is actuated to rotate the rotary control valve 254 via thedirectional gear 118 in a first direction, oil passes through a forwardorifice 366 (defined by the openings 314, 330 of plates 282, 286) of therotary control valve 254 to the motor 162. Oil is stored in thehydraulic fluid reservoir 78, or the sump, defined by the first chamberof the module 14. Oil is drawn from the sump 78, through the filter 102,to the fixed displacement pump 98. The oil is then pumped to the rotarycontrol valve 254 through the valve inlet 202 (represented by point A inFIG. 12). The pressure relief valve 110 relieves pressure within thehigh pressure fluid passage 106 (FIG. 4) by releasing oil to the sump 78when the system pressure exceeds a set limit, which is higher than thepressure expected during normal operations of the lawn tractor 10.

In the illustrated embodiment, the rotary control valve 254 is actuatedin a first direction to allow forward operation of the module 14 whenthe output demand of the motor 162 is less than a maximum output. Aportion of the oil flows through the forward orifice 366 of the rotarycontrol valve 254 and a portion of the oil flows through a bypass flowpath 370. For example, if output demand is 50 percent of the maximumoutput, 50 percent of the oil will flow through the forward orifice 366and 50 percent of the oil will flow through the bypass flow path 370. Ina further embodiment, when output demand is at the maximum output, thebypass flow path 370 is completely blocked by the flow compensatingvalve 114, and 100 percent of the oil will flow through the forwardorifice 366.

After flowing through the forward orifice 366, the oil is pumped throughthe motor 162 in the forward direction such that the motor 162 rotatesin the first direction, and thereby rotates the motor shaft 62 in theforward direction. The oil is then discharged back to the rotary controlvalve 254, where the oil flows through a reverse orifice 374 (defined bythe openings 318, 334 of plates 282, 286) of the rotary control valve254 and exits to the sump 78 through the valve outlet 278 (representedby point B in FIG. 12).

As the flow of high pressure oil passes through the forward orifice 366,a portion enters the transverse passage 900, pushes the ball 910 to thereverse orifice seat 905 and flows to the second space 210B of the flowcompensating valve 114. A portion of the high pressure flow in the highpressure flow path 106 flows along a bypass flow path 370 before thehigh pressure oil enters the rotary control valve 254. A small portionof this flow is directed to the first portion 210A of the flowcompensating valve 114. The interplay between the pressure in the firstportion 210A and the pressure in the second portion 210B controls themovement of the spool 234 in the flow compensating valve 114. Asillustrated in FIG. 12, a portion of the oil discharged by the pump 98is directed through the bypass flow path 370, through the flowcompensating valve, and into the sump 78.

FIG. 13 shows an exploded view of the motor 162 and the rotary controlvalve 254 in a neutral position, including the rotary plate 282 and thestationary plate 286. In the neutral position, oil from the pump 98 ispumped through the housing wall 270 to the inlet 202. However, therotary plate 282 blocks this opening and inhibits flow. In addition, thevent opening 920 is positioned adjacent the notch 281 to relieve anypressure in the second space 210B. Thus, the spool 234 moves to thebypass position to allow direct flow from the high pressure fluidpassage 106 back to the housing chamber 78 without forcing the oilthrough the rotary valve 254 or motor 162.

In one construction, maximum output of the motor 162 in a forwarddirection is greater than maximum output of the motor 162 in a reversedirection to ensure the maximum forward speed is greater than themaximum reverse speed. In order to cause the transmission to operateslower in the reverse direction than in the forward direction, therotary control valve 254 (i.e., the rotary plate 282) can be rotatedabout 9 degrees in a counter-clockwise direction. As discussed below,maximum forward output occurs at 30 degrees rotation in thecounter-clockwise direction, which is 39 degrees from mechanicalneutral, and maximum reverse output occurs at 30 degrees rotation in theclockwise direction, which is 21 degrees from mechanical neutral. Itshould be readily apparent to those of skill in the art that in otherembodiments the initial offset and angles of rotation may vary dependingon the size and configuration of the rotary control valve plates 282,286. For example, one construction is arranged such that 60 percent ofthe rotation of the rotary control valve 254 is in the forward directionrange (i.e., varies the speed in the forward direction) while 30 percentof the rotation of the rotary control valve 254 is in the reversedirection range (i.e., varies the speed in the reverse direction). Theremaining 10 percent of the rotation is in the neutral range and doesnot produce a speed change.

FIGS. 12, 14A, and 14B illustrate the motor 162 and the rotary controlvalve 254 in a forward, 50 percent output position (forward-50position), including the rotary plate 282 and the stationary plate 286.In the forward-50 position, the rotary control valve 254 is actuated toallow the motor 162 to operate in a forward direction at 50 percent ofthe maximum output demand. The rotary plate 282 is rotated in acounter-clockwise direction by the directional gear 118 (FIG. 7B) suchthat a portion of the forward opening 314 of the rotary plate 282overlaps the first opening 330 of the stationary plate 286 and the valveinlet 202. In this position, a portion of the reverse opening 318 of therotary plate 282 overlaps the second opening 334 of the stationary plate286 and the valve outlet 278. In the illustrated embodiment, the rotaryplate 282 is rotated about 10.5 degrees in a counter-clockwise directionfrom the neutral position (19.5 degrees from mechanical neutral) toachieve 50 percent output demand, with other rotational angles alsobeing possible.

FIG. 14A shows oil flow through the rotary control valve 254 to themotor 162, and FIG. 14B shows oil flow back through the rotary controlvalve 254 from the motor 162. Referring to FIG. 14A, oil enters theforward opening 314 of the rotary plate 282 from the valve inlet 202(FIG. 7B). Overlap portion 379 between the forward opening 314 and theinlet 202 is shown by a shaded area. Oil fills the forward opening 314of the rotary plate 282 and enters the first opening 330 of thestationary plate 286, which overlaps the forward opening 314. Overlapportion 380 between the forward opening 314 and the first opening 330are shown by a shaded area.

After filling the first opening 330 of the stationary plate 286, the oilenters the motor 162 and is pumped through the motor 162 to causerotation of the motor shaft 62. Referring to FIG. 14B, oil dischargedfrom the motor 162 enters the second opening 334 of the stationary plate286, which is shown by a shaded overlap portion 381. The oil fills thesecond opening 334 of the stationary plate 286 and then exits to thereverse opening 318 of the rotary plate 282, which overlaps the outlet278. Overlap portion 382 between the second opening 334 and the reverseopening 318 is shown by a shaded area. Oil fills the reverse opening 318of the rotary plate 282 and enters the valve outlet 278 to the sump 78.

In operation, about 50 percent or another portion of the oil bypassesthe motor 162 and returns to the sump 78 via the flow compensation valve114. It should be readily apparent to those of skill in the art that thediscussion with respect to the forward-50 position is applicable toother forward output demands less than the maximum output, i.e., whenthe rotary plate 282 is rotated in a counter-clockwise direction fromthe mechanical neutral position between 0 degrees and 30 degrees.

FIG. 15 is a hydraulic circuit diagram illustrating oil flow through thehydraulic transmission module 14 during reverse operation, which issimilar to the hydraulic circuit during forward operation. Like elementsto those described above with respect to FIGS. 1-12 are referenced bythe same reference numerals. Much of FIG. 15 is identical to FIG. 12, assuch, these portions will not be discussed in detail. The rotation ofthe rotary plate 282 in the opposite direction as it rotates in theillustration of FIG. 12 results in the inlet 202 directing oil throughthe reverse orifice 374, through the motor 162 and back through theforward orifice 366. Because the high pressure oil now passes throughthe reverse orifice 374 first, the high pressure oil forces the ball 910to the forward side seat 905 such that the high pressure fluid stillfills the transverse aperture 900. Thus, the present system is able tooperate in both forward and reverse by simply rotating the rotary plate282 in the opposite direction.

FIGS. 16A and 16B illustrate the motor 162 and the rotary control valve254 in a reverse, 50 percent output position (reverse-50 position),including the rotary plate 282 and the stationary plate 286. In thereverse-50 position, the rotary control valve 254 is actuated to allowthe motor 162 to operate in a reverse direction at 50 percent of themaximum output demand. The rotary plate 282 is rotated in a clockwisedirection by the directional gear 118 (FIG. 7B). A portion of thereverse opening 318 of the rotary plate 282 overlaps the second opening334 of the stationary plate 286 and the valve inlet 202, and a portionof the forward opening 314 of the rotary plate 282 overlaps the firstopening 330 of the stationary plate 286 and the valve outlet 278. In theillustrated embodiment, the rotary plate 282 is rotated about 19.5degrees in a clockwise direction from the neutral position (i.e., 10.5degrees from mechanical neutral) to achieve 50 percent output demand.

FIG. 16A shows oil flow through the rotary control valve 254 to themotor 162, and FIG. 16B shows oil flow back through the rotary controlvalve 254 from the motor 162. Referring to FIG. 16A, oil enters thereverse opening 318 of the rotary plate 282 from the valve inlet 202(FIG. 7B). Overlap portion 385 between the inlet 202 and the reverseopening 318 is shown by a shaded area. Oil fills the reverse opening 318of the rotary plate 282 and enters the second opening 334 of thestationary plate 286, which overlaps the reverse opening 318. Overlapportion 386, between the reverse opening 318 and the second opening 334,is shown by a shaded area.

After exiting the second opening 334 of the stationary plate 286, theoil enters the motor 162 and is pumped through the motor 162 to causereverse rotation of the motor shaft 62. Referring to FIG. 16B, oildischarged from the motor 162 enters the first opening 330 of thestationary plate 286, which is shown by a shaded overlap portion 387.The oil fills the first opening 330 of the stationary plate 286 and thenexits to the forward opening 314 of the rotary plate 282, which overlapsa portion of the first opening 330. Overlap portion 388, between thefirst opening and the forward opening 330, is shown by a shaded area.Oil fills the forward opening 314 of the rotary plate 282 and enters thevalve outlet 278 to the sump 78 such that oil exits the rotary plate 282to the sump 78, as shown by the arrow 384.

In operation of the transmission, a portion of the oil, about 50 percentin the illustrated embodiment, bypasses the motor 162 and returns to thesump 78 via the flow compensating valve 114. It should be readilyapparent to those of skill in the art that the discussion with respectto the reverse-50 position is applicable to other output demands lessthan the maximum output, i.e., when the rotary plate 282 is rotated in aclockwise direction from the mechanical neutral position between 0degrees and 30 degrees.

It should be noted that the illustrated construction includes asubstantially fixed volume housing that contains the various componentsas well as the oil. During operation of the transmission, the oilbecomes heated and expands. To accommodate the expansion, the housingcan be made slightly larger or an expansion tank or expansion chambercan be attached to the housing. In arrangements that employ anattachable tank, it is convenient to attach the tank to a fill port oropening such as fill port 990 illustrated in FIG. 3.

As illustrated in FIGS. 1 and 2, the illustrated hydrostatictransmission 14 drives the differential 18 that in turn drives twowheels 46 of the vehicle 10. One possible differential 18 suitable foruse with the illustrated transmission 14 is shown in FIGS. 19-23.

The drive shaft 22 extends between the transmission 14 and thedifferential 18 and provides a rotational connection between thetransmission 14 and the differential 18. The shaft 22 includes a firstend 400 coupled to the transmission 14 and a second end 405 coupled tothe differential 18. The first end 400 may also include a fan 410 thatrotates with the shaft 22 to direct cooling air onto the transmission 14and/or the differential 18.

With reference to FIG. 19, the first end 400 includes a hollow portion415 that defines a substantially rectangular or square cross section.The transmission 14 includes a motor shaft 62 that is substantiallysolid and defines a smaller rectangular or square cross section thatfits within the hollow portion 415. A rectangular or square bushing 420includes an inner surface 425 that engages the motor shaft 62 and anouter surface 430 that engages an inner surface 435 of the shaft 22. Thebushing 420 interconnects the motor shaft 62 and the shaft 22 forrotation.

In preferred constructions, the second end 405 is substantially similarto the first end 400, and a second bushing (similar to bushing 420)interconnects the shaft 22 and a differential input shaft 440. It shouldbe noted that while a rectangular or square bushing 420, motor shaft 62,and shaft hollow portion 415 are illustrated, other shapes could beemployed. For example, an oval or elliptical shape could also beemployed if desired. So long as the shape is capable of transmittingtorque between the interconnected shafts, the shape could be employedfor a bushing, motor shaft, and the drive shaft hollow portion.

FIG. 20 illustrates the first bushing 420 in greater detail. Inpreferred constructions, the second bushing is substantially the same asthe first bushing 420. However, different sizes or shapes could beemployed for the first or second bushing if desired.

The inner surface 425 of the bushing 420 is substantially square suchthat it can closely engage the square motor shaft 62. Once engaged withthe motor shaft 62, the square bushing 420 is inhibited from rotatingrelative to the motor shaft 62. Similarly, the outer surface 430 of thebushing 420 is square such that it closely engages the square innersurface 435 of the shaft 22. Thus, the bushing 420 is inhibited fromrotating relative to the shaft 22. In this way, the bushing 420 couplesto two shafts 62, 22 together to transfer torque therebetween.

The outer surfaces 430 of the bushing 420 are slightly curved in theaxial direction. The curve allows for slight radial and axialmisalignment between the two shafts 22, 62. Thus, in addition totransferring torque, the bushing 420 allows the shafts 22, 62 to beslightly misaligned or to change alignment slightly during vehicleoperation. Furthermore, the use of the bushing 420 and a substantiallyhollow shaft 22 allows for variation in the axial distance between thetransmission 14 and the differential 18. Specifically, if thedifferential 18 is slightly closer to the transmission 14, differentlengths of the drive shaft 22 and the differential shaft 440 extend intothe hollow portion 415 of the shaft 22. The use of an open bushing 420such as the one illustrated in FIG. 20 allows for variations in axialspacing as well as axial misalignment.

FIG. 21 illustrates one possible differential 18 suitable for use withthe vehicle 10. The differential 18 includes a housing 445 that at leastpartially supports the input shaft 440 and two axles 450 that extendsubstantially normal to the input shaft 440 and support two wheels 46.In the illustrated construction, the housing 445 is formed from a firstpiece 455 and a second piece 460 that bolt together to define an innerchamber 465. Generally, the housing 445 is cast from a metallic materialsuch that the housing 445 provides some level of protection for thecomponents inside. However, other manufacturing methods (e.g., forging,welding, machining, etc.) as well as combinations of manufacturingmethods are also suitable for use in manufacturing the housing 445.Although a metal housing is preferred, (e.g., cast aluminum, cast iron,cast steel, stainless steel, and the like) other materials may also besuitable for use.

FIG. 22 shows the differential 18 of FIG. 21 with the first piece 455 ofthe housing 445 removed to better illustrate the components disposedwithin the inner chamber 465. As can be seen, the first piece 455provides rotational support for the differential input shaft 440. Inmost constructions, one or more bearings are positioned between thehousing 445 and the input shaft 440 to facilitate smooth rotation of theinput shaft 440. A first bevel gear 470 is supported at the end of theinput shaft 440 such that the first bevel gear 470 rotates at the samespeed as the input shaft 440.

The first bevel gear 470 engages a second bevel gear 475 that is largerthan the first bevel gear 470. The larger second bevel gear 475 providesa first stage of speed reduction in the differential 18. The secondbevel gear 475 is supported for rotation by a first differential shaft480 that is substantially normal to the input shaft 440. The firstdifferential shaft 480 is supported for rotation by the first piece 455of the housing 445. As with the input shaft 440, most constructionsinclude one or more bearings disposed between the first piece 455 andthe first differential shaft 480 to facilitate efficient rotation.

With reference to FIGS. 21 and 22, a brake disk 485 is supported by thefirst differential shaft 480 and is disposed outside of the housing 445to facilitate engagement by a brake member 490. The brake member 490 mayinclude one or more calipers 495 coupled to the housing 445 or to thevehicle 10 that are used by the rider to slow the vehicle 10.

Returning to FIG. 22, the first differential shaft 480 also supports afirst gear 500. Because of the first stage of speed reduction, the firstgear 500, which rotates at the same speed as the first differentialshaft 480, rotates slower than the differential input shaft 440. Asillustrated, the first gear 500 is a spur gear. However, otherconstructions may employ helical or other gear arrangement as desired.

A second gear 505 engages the first gear 500 and rotates in response torotation of the first gear 500. As shown in FIG. 22, the second gear 505is much larger than the first gear 500 to provide a second stage ofspeed reduction. The second gear 505 is also illustrated as a spur gear,with other gears being suitable for use. The second gear 505 issupported on a second differential shaft 510 that is supported forrotation by the first and second pieces 455, 460 of the housing 445. Inmost constructions, the second differential shaft 510 is supported bythe housing 445, and gears 505, 515 rotate with respect to the shaft510.

The second differential shaft 510 is substantially parallel to the firstdifferential shaft 480 and is disposed slightly below the firstdifferential shaft 480. As illustrated in FIG. 22, the axis of thesecond differential shaft 510 is positioned near the interface betweenthe first piece 455 and the second piece 460 of the housing 445.

A third gear 515 is attached to the second differential shaft 510 and ispositioned adjacent the second gear 505. The third gear 515 is a spurgear that is smaller than the second gear 505. The third gear 515 isfixedly attached to the second differential shaft 510 such that thethird gear 515 and the second gear 505 rotate in unison. In addition,because the third gear 515 is fixedly attached to the seconddifferential shaft 510, the third gear cannot move axially along thesecond differential shaft 510.

FIG. 23 illustrates the arrangement of the second gear 505 and the thirdgear 515, with the second gear 505 illustrated in phantom. The secondgear 505 includes a central aperture 520 sized and shaped to receive theteeth of the third gear 515. When the teeth of the third gear 515 engagethe aperture 520 of the second gear 505, the second gear 505 becomesrotationally locked to the third gear 515 and the second differentialshaft 510 such that rotation of the second gear 505 produces acorresponding rotation of the third gear 515. However, the arrangementis such that the second gear 505 can move axially along the seconddifferential shaft 510 with respect to the third gear 515, whileremaining engaged for rotation. This arrangement eliminates the need fora splined or other shaft that allows for axial movement of one of thetwo gears 505, 515 on the second differential shaft 510.

Returning to FIG. 23, the third gear 515 engages a fourth, or ring gear525 that is larger than the third gear 515. The sizing of the third gear515 and the ring gear 525 results in a third stage of speed reduction.The ring gear 525 includes spur gear teeth on an outer surface of a ringthat defines a substantially hollow ring interior 530.

In some constructions, the ring gear 525 includes a shoulder 535 (FIG.23) that engages a corresponding shoulder 540 formed as part of thehousing 445. The engaged shoulders 535, 540 act as a bearing (i.e., ajournal bearing) that supports the ring gear 525 for rotation about aring gear axis 545. In other constructions, other support systems areemployed to support the ring gear 525 as will be discussed below.

Two shafts 550 extend toward one another along a diameter of the ringgear 525 within the ring interior 530 and support two ring bevel gears555 for rotation. Each of the ring bevel gears 555 is rotatably attachedto one of the shafts 550 such that the ring bevel gears 555 are free torotate about or with their respective shafts 550. In most constructions,bearings support the bevel gears 555 on the shafts 550 within the ringgear 525 to facilitate smooth reduced friction rotation.

The two axles extend from the housing 445 and support wheels 46 (shownin FIG. 1) that in turn support the vehicle 10. The axles 450 extendalong the ring gear axis 545 and are substantially parallel to the firstdifferential shaft 480 and the second differential shaft 510. An outerbearing 560, positioned between the housing 445 and the respective axle450 near the point where the axle 450 exits the housing 445, at leastpartially supports each axle 450 for rotation. Inner bearings (notshown) may be employed to further support each axle 450 in constructionsthat do not employ a shoulder 535 on the ring gear 525. In theseconstructions, the inner bearings are positioned between the housing 445and the axle 450 near the inner chamber 465.

Each axle 450 includes an axle bevel gear 565 disposed at its inner endand engaged with the ring bevel gears 555. In the illustratedconstruction, the axle bevel gears 565 are substantially the same sizeas the ring bevel gears 555. Of course, other sizes and gear types maybe suitable.

The ring bevel gears 555 rotate with the ring gear 525, but do notrotate about the ring gear shafts 550 during straight travel of thevehicle 10. Rotation of the ring bevel gears 555 with the ring gear 550causes rotation of the axle bevel gears 565 and rotation of the vehiclewheels 46. During a turn, the inner wheel 46 rotates more slowly thanthe outer wheel 46. To facilitate this, the ring bevel gears 555 rotateabout the ring shafts 550, thereby allowing the axle bevel gear 565associated with the inner wheel 46 to rotate slower than the axle bevelgear 565 associated with the outer wheel 46.

In most constructions, the cavity or inner chamber 465 is filled, orpartially filled with a lubricant, such as oil. The lubricant reduceswear of the interfacing components and gears and transfers excess heataway from these components. In addition, the differential 18 has beendescribed as including several bearings. Journal, needle, roller, ball,tapered roller bearings, and the like could be used for any or all ofthe bearings described.

In operation, the transmission provides power to the motor output shaft62 in the form of a torque at an output speed. The torque is transferredfrom the motor output shaft 62 to the drive shaft 22 via the bushing420. Any shaft misalignment is accommodated by the bushing 420. Thetorque is then transferred to the differential input shaft 440 such thatthe differential input shaft 440 rotates at a first speed substantiallyequal to the speed of the motor output shaft 62. Again, a second bushing420 between the drive shaft 22 and the differential input shaft 440accommodates minor axial misalignments, while efficiently transmittingtorque between the shafts 22, 440.

The rotation of the differential input shaft 440 causes the first bevelgear 470 to rotate at the same speed as the differential input shaft440. The first bevel gear 470 is engaged with the second bevel gear 475and causes a corresponding rotation. Because the second bevel gear 475is larger than the first bevel gear 470, the second bevel gear 475rotates at a second speed that is lower than the first speed. In theillustrated construction, the second bevel gear 475 is approximatelytwice the diameter of the first bevel gear 470, thus producing a speedreduction of approximately one-half.

The first spur gear 500 rotates at the second speed with the secondbevel gear 475 and engages the second spur gear 505. The second spurgear 505 is larger than the first spur gear 500, thus producing anotherspeed reduction. In the illustrated construction, the second spur gear505 is approximately three times the diameter of the first spur gear500. As such, the second spur gear 505 rotates at a third speed that isapproximately one-third the second speed.

The third spur gear 515 is rotationally coupled to the second spur gear505 such that the third spur gear 515 rotates at the third speed. Thethird spur gear 515 engages the ring gear 525 such that the ring gear525 rotates in response to rotation of the third spur gear 515. The ringgear 525 is larger than the third spur gear 515, thus producing a thirdstage of speed reduction. In the illustrated construction, the ring gear525 has a diameter that is approximately four times the diameter of thethird spur gear 515. Thus, the ring gear 525 rotates at aboutone-quarter the speed of the third spur gear 515.

The illustrated construction provides a speed reduction of about 24to 1. Thus, when the differential input shaft 440 rotates at 2400 rpm,the ring gear 525 rotates at about 100 rpm. In addition to the reductionin speed, there is a corresponding increase in torque at the ring gear525.

During straight-line operation of the vehicle 10, rotation of the ringgear 525 produces a corresponding rotation of the ring bevel gears 555.However, the ring bevel gears 555 do not rotate about the ring gearshafts 550. As such, the ring bevel gears 555 couple the axle bevelgears 565 to the ring gear 525 such that the axle bevel gears 565 rotateat substantially the same speed as the ring gear 525. In addition, theaxles 450 and the wheels 46 attached to the axles 450 rotate atsubstantially the same speed as the ring gear 525.

During a turn, one of the wheels 46, axles 450, and axle bevel gears 565must rotate slightly slower than the opposite wheel 46, axle 450, andaxle bevel gear 565. To facilitate this, the ring bevel gears 555 rotateabout the ring shaft axis 545. The rotation of the ring bevel gears 555allows one axle bevel gear 565 to rotate slower than the ring gear 525,while simultaneously allowing the opposite axle bevel gear 565 to rotatefaster.

While the illustrated construction includes spur gears and bevel gears,one of ordinary skill in the art will realize that other types of gears(e.g., helical, etc.) could be employed. Furthermore, additionalcomponents not described herein may also be included in the transmission14 or differential 18.

Various features and advantages of the invention are set forth in thefollowing claims.

1. A transmission for a vehicle, the transmission comprising: a fixeddisplacement pump operable to produce a high pressure flow; a fixeddisplacement motor; a fluid flow path between the pump and the motor,the fluid flow path integrated into a housing containing at least one ofthe pump and the motor; and a single control valve for controlling thedirection and the speed of the motor.
 2. The transmission of claim 1,wherein the fixed displacement pump includes a fixed displacementgerotor.
 3. The transmission of claim 2, wherein the fixed displacementmotor includes a fixed displacement gerotor.
 4. The transmission ofclaim 1, wherein the control valve includes a rotary valve plate and astationary valve plate.
 5. The transmission of claim 4, wherein thestationary plate includes a first aperture in fluid communication with afirst side of the motor and a second aperture in fluid communicationwith a second side of the motor.
 6. The transmission of claim 5, whereinthe rotary valve plate includes a first movable aperture and a secondmovable aperture positioned such that rotation of the rotary valve platein a first direction establishes a first fluid flow path from a highpressure inlet to an outlet to produce forward rotation of the motor,the first fluid flow path passing in order through the first movableaperture, the first aperture, the first side of the motor, the secondside of the motor, the second aperture, and the second movable aperture,and rotation of the rotary valve plate in a second direction establishesa second fluid flow path from the high pressure inlet to the outlet toproduce reverse rotation of the motor, the second fluid flow pathpassing in order through the second movable aperture, the secondaperture, the second side of the motor, the first side of the motor, thefirst aperture, and the first movable aperture.
 7. The transmission ofclaim 1, further comprising a flow compensating valve at least partiallyoperable in response to a first pressure to direct a portion of the highpressure flow to a sump and the remainder of the high pressure flow tothe motor.
 8. The transmission of claim 7, wherein the flow compensatingvalve includes a biasing member positioned to bias the flow compensatingvalve toward a first position in which no flow is directed to the sump.9. The transmission of claim 8, wherein the first pressure issubstantially equal to a pump discharge pressure, and wherein the firstpressure is directed to the flow compensating valve to bias the flowcompensating valve toward a second position in which all of the highpressure flow is directed to the sump.
 10. The transmission of claim 9,wherein a second pressure is directed to the flow compensating valve tobias the flow compensating valve toward the first position, the secondpressure being substantially equal to a pressure of a signal flow offluid from the control valve.
 11. The transmission of claim 10, whereinthe control valve, in a neutral position, defines a flow path thatdirects the signal flow of fluid to the sump such that the flowcompensating valve is biased to the second position and all of the highpressure flow is directed to the sump.
 12. The transmission of claim 7,wherein the motor, the pump, the control valve, and the flowcompensating valve are disposed in the housing.
 13. The transmission ofclaim 7, wherein the housing defines a portion of the control valve, theflow compensating valve, and the fluid flow path between the pump andthe motor.
 14. The transmission of claim 1, wherein the motor has amotor shaft and the pump has a pump shaft, the pump shaft being normalto the motor shaft.
 15. A hydrostatic transmission for a vehicle, thehydrostatic transmission comprising: a fluid pump; a fluid motor; arotary control valve configured to oscillate independent of the movementof the pump and the motor; and a single control interface configured tocontrol both a speed and a direction of the motor.
 16. The hydrostatictransmission of claim 15, wherein the interface includes a leverconfigured to move in a linear path.
 17. The hydrostatic transmission ofclaim 15, further comprising a linkage between the rotary control valveand the interface.
 18. The hydrostatic transmission of claim 17, whereinthe valve includes a valve gear, and wherein the linkage includes alever and a second gear that engages the valve gear.
 19. The hydrostatictransmission of claim 18, wherein the valve gear includes a bevel gearaffixed to the valve, and wherein the second gear is a pinion gear. 20.The hydrostatic transmission of claim 15, wherein at least one of thepump and the motor has a fixed displacement.
 21. The hydrostatictransmission of claim 15, wherein both the pump and the motor have fixeddisplacements.
 22. The hydrostatic transmission of claim 15, wherein thepump is a gerotor type pump.
 23. The hydrostatic transmission of claim22, wherein the motor is a gerotor type motor.
 24. The hydrostatictransmission of claim 15, wherein the motor, the pump and the controlvalve are disposed in a common housing.
 25. The hydrostatic transmissionof claim 24, wherein the housing defines a portion of a fluid flow pathbetween the motor and the pump.
 26. The transmission of claim 15,wherein the rotary control valve includes a rotary valve plate and astationary valve plate.
 27. The transmission of claim 26, wherein thestationary plate includes a first aperture in fluid communication with afirst side of the motor and a second aperture in fluid communicationwith a second side of the motor.
 28. The transmission of claim 27,wherein the rotary valve plate includes a first movable aperture and asecond movable aperture positioned such that rotation of the rotaryvalve plate in a first direction establishes a first fluid flow pathfrom a high pressure inlet to an outlet to produce forward rotation ofthe motor, the first fluid flow path passing in order through the firstmovable aperture, the first aperture, the first side of the motor, thesecond side of the motor, the second aperture, and the second movableaperture, and rotation of the rotary valve plate in a second directionestablishes a second fluid flow path from the high pressure inlet to theoutlet to produce reverse rotation of the motor, the second fluid flowpath passing in order through the second movable aperture, the secondaperture, the second side of the motor, the first side of the motor, thefirst aperture, and the first movable aperture.
 29. The transmission ofclaim 15, further comprising a flow compensating valve at leastpartially operable in response to a first pressure to direct a portionof the high pressure flow to a sump and the remainder of the highpressure flow to the motor.
 30. The transmission of claim 29, whereinthe flow compensating valve includes a biasing member positioned to biasthe flow compensating valve toward a first position in which no flow isdirected to the sump.
 31. The transmission of claim 30, wherein thefirst pressure is substantially equal to a pump discharge pressure, andwherein the first pressure is directed to the flow compensating valve tobias the flow compensating valve toward a second position in which allof the high pressure flow is directed to the sump.
 32. The transmissionof claim 31, wherein a second pressure is directed to the flowcompensating valve to bias the flow compensating valve toward the firstposition, the second pressure being substantially equal to a pressure ofa signal flow of fluid from the control valve.
 33. The transmission ofclaim 32, wherein the control valve, in a neutral position, defines aflow path that directs the signal flow of fluid to the sump such thatthe flow compensating valve is biased to the second position and all ofthe high pressure flow is directed to the sump.
 34. The transmission ofclaim 29, wherein the motor, the pump, the control valve, and the flowcompensating valve are disposed in the housing.
 35. The transmission ofclaim 29, wherein the housing defines a portion of the control valve,the flow compensating valve, and the fluid flow path between the pumpand the motor.
 36. The hydrostatic transmission of claim 15, furthercomprising a flow compensating valve disposed downstream of the motor,wherein the flow compensating valve controls the speed of the motorbased on a pressure of a flow fluid discharged from the pump.
 37. Thehydrostatic transmission of claim 15, wherein the rotary control valveincludes a rotary valve plate configured to oscillate through a range ofless than 180 degrees independently of the pump and the motor.
 38. Thehydrostatic transmission of claim 29, wherein the rotary valve plate isconfigured to oscillate through a range of about 40-80 degrees.
 39. Atransmission comprising: a pump configured to discharge a flow of fluidat a first pressure; a motor configured to rotate in response to a flowof fluid in a first flow path; a first valve movable to vary the flow offluid in the first flow path; and a second valve movable between a firstposition and a second position at least partially in response to thefirst pressure to direct a portion of the flow of fluid to the firstflow path and a remainder of the flow of fluid to a second flow path,wherein as the valve moves toward the second position, additional flowis diverted from the second flow path to the first flow path to increasethe speed of the motor.
 40. The transmission of claim 39, wherein thepump, the first valve, the motor, and the second valve are disposed in acommon housing.
 41. The transmission of claim 40, wherein the housing atleast partially defines the first flow path and the second flow path.42. The transmission of claim 39, wherein the first valve controls adirection of fluid flow through the first flow path, and wherein themotor rotates in a first direction when the flow of fluid through thefirst flow path is in a first direction, and the motor rotates in asecond direction when the flow of fluid through the first flow path isin a second direction.
 43. The transmission of claim 39, furthercomprising an operator interface and a linkage between the interface andthe first valve, the operator interface configured to enable an operatorto control the first valve.
 44. The transmission of claim 39, wherein atleast one of the pump and the motor has a fixed displacement.
 45. Thetransmission of claim 39, wherein both the pump and the motor have afixed displacement.
 46. The transmission of claim 39, wherein the pumpis a gerotor type of pump, the motor is a gerotor type of motor, and thefirst valve is a rotary control valve.
 47. The transmission of claim 39,wherein the first valve includes a rotary valve plate and a stationaryvalve plate.
 48. The transmission of claim 47, wherein the stationaryplate includes a first aperture in fluid communication with a first sideof the motor and a second aperture in fluid communication with a secondside of the motor.
 49. The transmission of claim 48, wherein the rotaryvalve plate includes a first movable aperture and a second movableaperture positioned such that rotation of the rotary valve plate in afirst direction establishes a first fluid flow path from a high pressureinlet to an outlet to produce forward rotation of the motor, the firstfluid flow path passing in order through the first movable aperture, thefirst aperture, the first side of the motor, the second side of themotor, the second aperture, and the second movable aperture, androtation of the rotary valve plate in a second direction establishes asecond fluid flow path from the high pressure inlet to the outlet toproduce reverse rotation of the motor, the second fluid flow pathpassing in order through the second movable aperture, the secondaperture, the second side of the motor, the first side of the motor, thefirst aperture, and the first movable aperture.
 50. The transmission ofclaim 49, wherein the second valve includes a biasing member positionedto bias the second valve toward a first position in which no flow isdirected to the second flow path.
 51. The transmission of claim 50,wherein the first pressure is directed to the second valve to bias thesecond valve toward a second position in which all of the flow isdirected to the second flow path.
 52. The transmission of claim 51,wherein a second pressure is directed to the second valve to bias thesecond valve toward the first position, the second pressure beingsubstantially equal to a pressure of a signal flow of fluid from thefirst valve.
 53. The transmission of claim 52, wherein the first valve,in a neutral position, defines a flow path that directs the signal flowof fluid to a sump such that second valve is biased to the secondposition and all of the flow is directed to the second flow path.
 54. Ahydrostatic transmission module for use with a small engine, thehydrostatic transmission module comprising: a module housing having afirst chamber, and a second chamber in fluid communication with thefirst chamber; a fixed displacement pump disposed in the first chamberof the module housing, the fixed displacement pump operatively coupledto a crankshaft of the engine and configured to discharge a flow offluid having a first pressure; a fixed displacement motor disposed inthe second chamber of the module housing, the motor configured to rotatein response to the flow of fluid from the pump; a first valve operableto divide a flow of fluid from the pump into a first flow of fluid thatflows to the motor and a second flow of fluid that flows to a sump; anda second valve disposed in the first flow and movable to vary the firstflow of fluid to the motor.
 55. The hydrostatic transmission of claim54, wherein the pump is a gerotor type of pump, the motor is a gerotortype of motor, and the second valve is a rotary control valve.
 56. Thehydrostatic transmission of claim 54, wherein the pump, motor, firstvalve, and second valve are disposed in a single housing.
 57. Thehydrostatic transmission of claim 56, wherein the housing at leastpartially defines a first flow path for the first flow of fluid and asecond flow path for the second flow of fluid.
 58. The hydrostatictransmission of claim 54, further comprising an operator interface and alinkage between the interface and the second valve, the operatorinterface configured to enable an operator to control the second valve.59. The hydrostatic transmission of claim 58, wherein the linkageincludes a lever and a gear, and wherein the gear engages the valve. 60.The hydrostatic transmission of claim 54, wherein the second valveincludes a rotary valve plate and a stationary valve plate.
 61. Thehydrostatic transmission of claim 60, wherein the stationary plateincludes a first aperture in fluid communication with a first side ofthe motor and a second aperture in fluid communication with a secondside of the motor.
 62. The hydrostatic transmission of claim 61, whereinthe rotary valve plate includes a first movable aperture and a secondmovable aperture positioned such that rotation of the rotary valve platein a first direction establishes a first fluid flow path from a highpressure inlet to an outlet to produce forward rotation of the motor,the first fluid flow path passing in order through the first movableaperture, the first aperture, the first side of the motor, the secondside of the motor, the second aperture, and the second movable aperture,and rotation of the rotary valve plate in a second direction establishesa second fluid flow path from the high pressure inlet to the outlet toproduce reverse rotation of the motor, the second fluid flow pathpassing in order through the second movable aperture, the secondaperture, the second side of the motor, the first side of the motor, thefirst aperture, and the first movable aperture.
 63. The hydrostatictransmission of claim 54, wherein the first valve includes a biasingmember positioned to bias the first valve toward a first position inwhich no flow is directed to the second flow of fluid.
 64. Thehydrostatic transmission of claim 63, wherein the first pressure isdirected to the first valve to bias the first valve toward a secondposition in which all of the flow is directed to the second flow offluid.
 65. The hydrostatic transmission of claim 64, wherein a secondpressure is directed to the first valve to bias the first valve towardthe first position, the second pressure being substantially equal to apressure of a signal flow of fluid from the second valve.
 66. Thehydrostatic transmission of claim 65, wherein the second valve, in aneutral position, defines a flow path that directs the signal flow offluid to a sump such that first valve is biased to the second positionand all of the flow is directed to the second flow of fluid.
 67. Thehydrostatic transmission of claim 54, wherein the second valve controlsa direction of fluid flow to the motor.
 68. The hydrostatic transmissionof claim 55, wherein the motor rotates in a first direction when thefluid flow to the motor is in a first direction, and wherein the motorrotates in a second direction when the fluid flow to the motor is in asecond direction.